The insecticidal properties of 6,7,8,9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodi oxathiepin-3-oxide are already known (see German Pat. No. 1,015,797). It is also known that formamidine derivatives have an acaricidal and insecticidal effect (see German Patent 1,172,081 and Swiss Pat. No. 533,423).